Animate the Viewport Visibility of an Object within the Timeline?
Make your Object visible at frame 30:

You can keyframe the Viewport Visibility of an Object from within the Outliner:


In the Timeline select Frame 1
Then open the Outliner
Set your Object to Hidden
While the mouse hovers over the eye icon in the outliner, press I to keyframe the visibility.
Move on to frame 30
Click on the Eye icon again to make the object visible
Then press I again to keyframe the visibility change.




Note: If you want to achieve the same for your render, then please check the answer in here: How can I animate the Restrict Visibility property of an object?